In a typical expitaxial reactor, it is common to use an evacuated quartz bell jar to house the wafer on which deposition is desired and a supporting substrate adjacent to the wafer. The wafer and substrate are then irradiated by an external source, generally infrared radiation, to bring the wafer to the required temperature for the desired chemical reactions to occur. In the most common configurations, the wafer is typically of silicon, the substrate is of graphite or graphite coated with silicon carbide, and the wavelength typically used for heating the silicon is generally from about 2 to 10 micrometers.
Such a configuration is, however, very inefficient for the heating of silicon wafers, since in this range of infrared wavelengths, silicon is substantially transparent, exhibiting an absorption coefficient typically less than 100/cm, and more often less than about 10/cm. Considering this low coefficient of absorption, it is likely that a significant portion of the heating of the wafer occurs by conduction from the graphite substrate which acts as a good absorber, rather than by absorption of the infrared in the wafer itself. Because of this rather indirect heating process, reactor design can often become the fine art of choosing the proper substrate materials.
For technical and economic reasons, it would be very beneficial to be able to heat semiconductor wafers directly in a reactive gas stream without the use of a supporting substrate for conductively heating the wafer. In particular, it would enable chemical reactions to take place directly only at the surface of the heated wafer rather than on both the wafer and its heated substrate.